Thermostat constructions are well known in the art. For example, such constructions are shown in U.S. Pat. No. 2,497,397, Dales, issued Feb. 14, 1950 and U.S. Pat. No. 3,100,247, Dales, issued Aug. 6, 1963. Each of these patents, as many of the others in the field, describes a thermostatic construction involving one or more bimetallic arms enclosed within a metallic casing. At least two contacts are provided, the contacts being joined when a bimetallic arm is in one position, and being separated, to break the contact, when a bimetallic arm is in the opposite position.
The constructions shown in the referenced U.S. Pat. Nos. 2,497,397 and 3,100,247 are not meant to be limiting, but merely illustrative of thermostat constructions employing bimetallic spring arms within a metallic casing. In these constructions, an insulating material is necessary between the casing and the point at which the contact arm, or a connector member attached to that contact arm, passes through the end of the casing. Among other things, the insulating member, or sleeve, is meant to prevent the free passage of air to and from the operating portion of the thermostat within the casing.
As shown in the two referenced patents, braided fiberglass is a frequently used insulating material in these constructions. Similarly, as referred to in the '397 patent, mica is often used as an insulating material. However, this fiberglass insulation, though widely employed in these constructions, presents several problems.
For example, the fiberglass is not dimensionally stable, and will expand as the ambient temperatures surrounding the thermostatic element increases. Further, in order to obtain sufficient dielectric strength in the fiberglass member, it must generally be impregnated. Continued operation of the thermostatic element, particularly in high temperature environments, tends to cause the material used for impregnation to migrate from the fiberglass member to the operating components of the thermostat. This contaminates the operating parts of the thermostat, impairing its accuracy and, frequently, causing premature wear of the parts. While the mica is frequently employed to lessen the problem caused by the low degree of imperviousness of the fiberglass, it does not completely solve the problem, as the fiberglass must still be used.
One method for avoiding the problems caused by fiberglass and/or mica has been to replace the fiberglass and mica with a polyimide film, such as that sold by E. I. duPont deNemours & Co. under the trademark "Kapton." This is not an entirely effective solution, either, however. The material is generally placed between the metallic casing and the contact strips in the form of a tube. Because Kapton cannot be formed with a sufficient thickness, multiple concentric tubes are generally employed. Because these are difficult to automatically index, thermostatic devices with Kapton insulation cannot generally be formed by the most expeditious method of manufacture, automatic assembly machines.
While the standard nylons have many of the characteristics desirable for a thermostatic device insulation, they, cannot be used as they will not withstand the high temperature environments in which such thermostatic devices are normally placed. While aromatic polyimides have been employed for their dielectric strength as shown, for example, in U.S. Pat. No. 4,259,544, Litauer, they have not been employed in thermostats, nor in the type of environment in which thermostats are called upon to operate.